2011 Prius 281k miles - would you replace the head gasket if you were in my situation?

Yeah lack of EGR flow contributes to the condition assuming non inert air will take its place leaning it out on individual cylinders at cylinders 1 and 2 typically since they lose flow first due to clogging of the egr channel in the intake since they are the furthest. I've done spark plugs 3 times now and you can see cylinders 1 and 2 always had compression charge blowing around the compression washers on the spark plug getting onto the threads which means there was probably extraordinary pressure in those cylinders at times due to misfire/pinging, whereas 3 and 4 always had none. Also, they had the most wearing on the electrodes. Knock threshold wise toyota programs a limit and they seem to not set the limit off to give a misfire code until its too late. AFR wise toyota only goes by bank so the afr won't notice outside of what comes out of all 4 cylinders combined in the bank gives the front oxygen sensor (afr sensor).
Typically if say the engine on a v8 starts up, goes into closed loop after initial check, and you drive around it logs a misfire x times on cylinder(s) x, it will set the ecm to go to open loop to try and correct the condition with live data which doesn't always work. AFR wise it will just say bank 1 or bank 2 lean (left or right sides) and then also go into open loop to try and correct and not blow up. Its programmed to the best of its abilities by very smart people, but its not able to be perfect.
If it was say a racing engine with sensors on each cylinder it could probably do more, but this is very expensive and complex and not really able to be done on passenger cars. They typically just do it in development and then swap to a single sensor in each collector.

 

Is this answering why the Prius knock sensor does not keep the knock away..... I missed it....

 

This part I understand. EGR is 'inert air', correct?
So these 'egr blocked' cylinders will get more air and less thinning with EGR gasses which results in a leaner mixture just in those cylinders,
correct?

Now if the EGR is not working, due to an unplugged EGR valve or a block off plate, all the cylinders would/should get an even mixture.
But wouldn't that mixture still be controlled by the O2 sensor?

And, a code for EGR is set when MAF and MAP don't see the expected difference due to no added EGR flow, correct?
(But since the total EGR flow is still getting into the intake, just unevenly and resulting in big difference in mixtures, this code may not work until all the EGR ports are so clogged that there is an 'expected' MAF/MAP discrepancy, correct?)

Is it the same code as an unplugged EGR valve?
(Where the command to open is not seen on the feedback sensor, correct?)

ps.
I could test these questions myself, but my poor old Prius mainly sits, unloved, for weeks at a time. Why burn that smelly stuff when there's an EV available?
I did show it some love this week, I put my expensive 2A battery minder (with the anti-sulfation function) on it for a few days. It needed it...
The 12V battery shows a date code of 3 - 17 and yet it is hanging in there!

 

When your car is just sitting I’d leave it on the charger all the time, if practical:

^ Yes. It’s typically the intake port for cylinder one, then two, and so on, that clog first. And the head gasket failures are typically at the adjoining wall between cylinder one and two.

 

So what are your thoughts on 'defeating' the EGR function?
If it's only a '3 -4%' hit to mpg, I could live with that.
That's a typical range of error with every tank fill.
If I had consistent driving patterns it may be more measurable, but I don't.
Lately it's only been long road trips, like to see biggest dam rocket ever made blow up after 4 minutes in the air.

Think of all the time and expense savings of not dikin under the hood 'every 50k miles', as you prescribe...

 

Yeah the car can't detect the imbalance very well in individual cylinders hence the head gasket failure in the same cylinder every time. It only knows if all 4 are lean or out of spec. But that happens way after the individual cylinders start having issues. Yeah EGR is inert gas, so the car basically trys to run like a diesel by keeping the throttle valve almost all the way open at all times to avoid the throttle loss and varies egr flow to control the throttling of the engine.
Without the egr plugged in, it just runs a different map without egr factored in. I wouldn't recommend a block off plate but i would assume that would get you a p0401. It may or may not run like the p0403 from simply unplugging the valve.

 

I prefer to keep it functional.

If I may "fearmonger" a bit: I think it's essential for the North American Prius, to avoid engine failure. The EGR system does double duty, it reduces emmisions and lets Toyota's engineers push things a little more. The latter I think is somewhat analogous to an interference engine: it's ok, as long everything's functioning as it should.

 

I would say for the most part the car detects the valve and cooler issues well and adjust pretty good for those lack of flows. The car just cannot detect the intake manifold clogging as its so uneven. I'm guessing here on this but I would suspect if you kept the intake manifold ports clean the car probably won't blow up and would pull a p0401 eventually letting you know its time to clean the egr valve and backward side. Keeping it clean works really well if your car doesn't burn a shit ton of oil. Burning oil is a very carbony and sludgy process compared to gas. It also doesn't help that the atkinson process blows oil vapor back into the intake manifold from the rings during the compression cycle as well.

 

I'm not burning any oil yet, but the carbon was building up nicely in the EGR.

 

The EGR system is also dumping carbon back into the engine. I wonder what detrimental effects that has? Carbon buildup is not a good thing, especially on the piston rings. Hey Mendel, how about inventing a carbon catch can?

 

Wait, I thought you were the guy saying you had the EGR unplugged and still got decent fuel mileage.
Do I need to try unplugging the EGR and see what code I get and how the car performs?
I have Toruque Pro and can clear codes. How quickly do these codes show up?

"push things a little bit more"? Like how much more? 3-4% in fuel economy?
Is that what you are implying?
I can live without that if it means not tearing into the engine every dang 50k miles to perform 10 hrs of maintenance work.
And especially if it means my HG might last like normal cars.

 

I unplugged and you get p0403. I still get insane gas mileage. There's not a large enough difference to be measured on my end. So unplugged it stays. Try it out. There's probably dozens at this point who have done it maybe more. I've only had so many tell me they also did it with good results.

 

If you disable the EGR the head gasket will benefit? Ok…

 

I believe so IF the intake, valves and pistons are not already carboned up. If they are full of carbon a head rebuild is needed. Realistically a full rebuild to reduce blowby and oil burning. The problem is an egr generated check engine light would remain on and could mask a future issue.

I would block the egr if I had a P10 style monitor for temperature display that also monitored for the code and then cleared it. But only that code. The P10 can scan and clear but would need some user code / logic capability.

 

Thanks! I was hoping to hear that!

Thanks, I was looking forward to your opinion on this subject!

I don't think anyone is doing a 'head rebuild' to clean off normal carbon build up these days. That's something that was prescribed to MG owners back in the 60's.
Modern engines don't get full of carbon. As for the intake manifold, I won't care if those tiny EGR tracks are clogging because they won't be used anymore, correct?
And nobody is doing a 'full rebuild' due to normal oil consumption. I know if it gets bad enough your cat can clog, but until then....adding a qt. every ?800 miles? is tolerable. My engine is in great shape and I don't add oil between 5k oil changes. I used to go 10k, especially in the summer months, but not so much anymore after reading about carbon clogging the oil control rings and drain holes. Although a piston soak has worked for a friends oil consuming Prius.

I have the Torquepro app and a 7" tablet I could keep mounted in the car. You can configure a page full of engine gauges and check and clear fault codes everyday if you like.

Now, to the car!!!
As I said, my usage is not consistent so it will be hard to get a good number on fuel economy with and without EGR

 

With my limited understanding:

I don’t think EGR cleaning will improve mpg, wouldn’t expect it. The EGR system allowed Toyotas engineers to push things a little further, get a slight mpg improvement. Those “pushes” are there, with or without functional EGR.

Except, without functional EGR those “pushes” go too far? The simplest EGR “delete” is to just let it fully clog up, which happens around 150~200k miles, around the same time that something else happens, with tedious regularity.

 

But the theory goes that clogging is uneven and can stress the head gasket.
Stopping EGR function at the valve makes for even flow to the cylinders. At least that's what I'm going for.
I can live with a minor mpg hit, if I can measure it.

 

If you've got deep pockets, go for it.

 

Not following. I have normal pockets.
Are you implying disconnecting the EGR valve will destroy my engine?
Shouldn't the Knock Sensor keep anything 'knock' related from happening?

 

Yes.